Assessment of Tribological Coatings for Foil Bearing Applications

A critical component in oil-free gas foil bearings is the tribological coating system that must be used on the journal/runner and the foil pads to ensure reliable operation during transient periods and start-stop cycles. The objective of this investigation was to evaluate the performance characteristics of potential coatings for use in foil bearing applications. Tribological tests were performed using a single foil thrust bearing pad in contact with a rotating flat disk at room temperature. The coatings consisted of a tungsten disulfide-based solid lubricant (KorolonTM 900), hydrogenated diamond-like carbon film (H-DLC), and thin dense chrome plating deposited by the ElectrolyzingTM process. The results demonstrated an excellent performance for the foil pads coated with the tungsten disulfide-based solid lubricant tested against either chrome-plated or H-DLC-coated disks. The very low friction that was expected for H-DLC, however, was not observed and the tungsten disulfide-based coating dominated the tribological behavior. It is concluded that the best coating system for foil bearing applications consists of placing the soft solid lubricant coating on the foil pad and the hard, wear-resistant coating on the disk. Other combinations provide high coefficients of friction and liftoff speeds and result in high levels of wear. This phenomenon is explained using the quasi-hydrodynamic theory of powder lubrication.